Apparatus for making absolute two-demensional position measurements

ABSTRACT

An apparatus is proposed for making absolute, two-dimensional measurements of the position of an object (10) with reference to a measuring arrangement (11), with the surface of the object (10) being configured as a hologram. The information contained in the hologram is red out by means of a measuring wave (13) that is directed onto the hologram (18), is diffracted there and interferes with a reference wave (14). The interfered-with waves (13, 14) are picked up by an optical sensor arrangement (20) including at least two sensors (21, 22). In a signal processing arrangement (23), the absolute position is determined either from the radiation intensities picked up by the sensors (21, 22) or from the phase relationship between the signals emitted by the sensors (21, 22), in each case by a comparison with a value stored in a memory.

STATE OF THE ART

The invention relates to an apparatus for making absolutetwo-dimensional measurements of the position of an object relative to ameasuring arrangement.

German Patent Application DE-P 3,821,046 discloses a method of measuringtravel and/or angles by opto-electronic means in which a first and asecond optical beam are directed at predetermined angles onto thesurface of an object to be measured and are diffracted on the surface ata common point of incidence. Both beams either have differentfrequencies or the frequency of the two beams changes simultaneously,with one of the beams being modulated in phase. The change in phase ofthe signal emitted by an optical sensor relative to a reference signalis evaluated. Diffraction takes place at the point of incidence of thetwo beams on the surface of the object to be measured because thesurface always has a certain roughness which has the effect of anoptical grating. The prior art method is able to perform an absoluteposition measurement if, at a certain measuring moment, for example atthe beginning of the measurement, a reference position is defined.However, an absolute measurement of the position of the object to bemeasured relative to the measuring arrangement on the basis of the justcompiled measured values is not possible.

It is the object of the invention to provide an apparatus for makingtwo-dimensional measurements of the position of an object relative to ameasuring arrangement so as to permit an absolute position measurementto be made from the just obtained measured values.

SUMMARY OF THE INVENTION

The above object is generally achieved according to the presentinvention by an apparatus for making absolute, two-dimensionalmeasurements of the position of an object with reference to a measuringarrangement wherein an optical measuring wave is directed onto a surfaceof the object, with the surface being configured as a hologram and anoptical reference wave is provided for interfering with the measuringwave diffracted by the hologram, an optical sensor arrangementcomprising at least two sensors is provided for detecting theinterfered-with waves, and a signal processing arrangement determinesthe position of the object from the output signals of the sensors andfrom a value stored in a memory.

ADVANTAGES OF THE INVENTION

The basic apparatus according to the invention as described aboveexhibits very high resolution at relatively little expense. Theconfiguration of at least one surface of the object to be measured as ahologram permits the accommodation of a great information density whichis evaluated by the simply configured optical measuring arrangement.

The information contained in the hologram is evaluated in that ameasuring wave is directed onto the hologram and a reference wave isprovided to interfere with the measuring wave diffracted by thehologram. The interfered-with waves are picked up by a sensorarrangement including at least two sensors whose output signals are fedto a signal processing arrangement which determines from these signalsand from values stored in a memory the absolute, two-dimensionalposition of the object with reference to the measuring arrangement.

According to a first embodiment of the apparatus according to theinvention the signal processing arrangement determines the absoluteposition from the sensor output signals which are proportional to theintensity of the interfered-with waves, for example by difference orquotient formation from the signals emitted by the at least two sensors.The significant advantage of this embodiment is the simple configurationof the optical arrangement because the measuring wave and the referencewave can both have the same frequency.

In a second embodiment it is provided that the signal processingarrangement determines the absolute position from the phaserelationships between the output signals emitted by the at least twosensors. In this embodiment it is necessary that a phase or frequencymodulation occurs between the measuring wave and the reference wave. Themodulation permits evaluation of the phase relationships at a frequencywhich is very low compared to the optical frequency of the measuring andreference waves and which can be evaluated by means of a very simpleelectronic arrangement. This embodiment has the particular advantagethat the intensities of measuring wave and reference wave are notincluded in the result of the absolute position measurement.Consequently, a greater measuring accuracy and/or resolution isattained.

An advantageous feature of the apparatus according to the inventionprovides that the measuring and reference waves are directed fromdifferent directions onto a common point of incidence on the hologram.Adjustment of the optical arrangement in this configuration is easy. Itmerely need be ensured that the measuring wave as well as the referencewave are directed onto the common point of incidence. Adjustment of theoptical sensor arrangement is particularly simple. It merely need beensured that the interfered-with waves which are diffracted at the pointof incidence impinge on the sensors. Small dimensions for the opticalmeasuring arrangement are possible due to the use of a semiconductorlaser as the radiation source, with the optical configuration preferablybeing effected in integrated form.

A further increase in resolution and measuring accuracy is possible ifthe optical sensor arrangement is configured as an array or as atwo-dimensional multi-sensor arrangement. In this configuration of theoptical sensor arrangement, the output signals of several sensors areevaluated in each case.

According to an advantageous modification of the apparatus according tothe invention, at least one further measuring wave and reference waveare provided in each case which have a different wavelength compared tothe already existing waves.

In a first embodiment of the modification, two further sensors are alsoprovided in the sensor arrangement, with both sensor groups eachdetecting one wavelength of the interfered-with waves. The division ofthe optical measuring arrangement into two parts of differentwavelengths permits simple realization of the hologram because, forexample, the one wavelength can be associated with the one dimension andthe other wavelength can be associated with the other dimension. Thecalculation and generation of the hologram then becomes particularlysimple.

As a further feature of the modification it is provided that thedifferent wavelengths are generated alternatingly in time. The low costof the sensor arrangement is a particular advantage since it requiresonly two sensors and the signal processing arrangement is able to makethe association with the different wavelength with the aid of a signalsupplied to it for this purpose.

Further improvements and modifications of the apparatus according to theinvention will become evident from further dependent claims inconjunction with the description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 depict two embodiments of an apparatus for makingabsolute, two-dimensional position measurements according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an object 10 whose absolute, two-dimensional position withreference to a measuring arrangement 11 is measured. The opticalmeasuring arrangement 11 includes a radiation source 12 from whoseradiation a measuring wave 13 and a reference wave 14 are derived. Themeasuring wave 13 as well as the reference wave 14 are conducted througha light waveguide 15. A modulator 16 is disposed in the beam path of thereference wave. The measuring wave 13 and the reference wave 14 aredirected onto a common point of incidence 17 on the surface 18 on object10, with the surface being configured as a hologram. The structure ofthe hologram is indicated by lines 19. The waves 13 and 14 diffracted atpoint of incidence 17 interfere with one another and are detected by anoptical sensor arrangement 20 which includes at least two sensors 21 and22. The output signals of sensors 21 and 22 are fed to a signalprocessing arrangement 23 which determines the position.

FIG. 2 shows an alternative configuration of the beam path of theapparatus shown in FIG. 1. The components that coincide with those shownin FIG. 1 bear the same reference numerals in FIG. 2 as in FIG. 1.Reference wave 14 is brought to be superposed on measuring wave 13 at apoint which lies between sensor arrangement 20 and hologram 18. A beamdivider 24 is provided to combine the two waves 13 and 14.

The apparatuses according to the invention shown in FIGS. 1 and 2operate as follows:

The information regarding the absolute two-dimensional position ofobject 10 with reference to the optical measuring arrangement 11 iscontained in the surface 18 of object 10, with the surface beingconfigured as a hologram. The hologram may also be produced separatelyand may then be connected with the surface of object 10. The structureof the hologram 18 can be calculated theoretically and is much morecomplicated in practice than sketched in simplified form as lines 19.The information contained in hologram 18 is read out by optical means.The measuring wave 13 directed onto hologram 18 is diffracted at pointof incidence 17. The position information is contained in the phaseposition of the diffracted wave and is evaluated relative to referencewave 14 which, for this purpose, is caused to interfere with measuringwave 13. According to FIG. 1, interference occurs in that measuring wave13 as well as reference wave 14 are directed onto the same point ofincidence 17. However, it is not necessary for reference wave 14 to alsoimpinge on hologram 18. According to FIG. 2, it may therefore also beprovided that reference wave 14 is caused to interfere with themeasuring wave 13 diffracted by the hologram only directly ahead of theoptical sensor arrangement 20. For example, reference wave 14 may bedirected directly onto the sensor arrangement 20. A beam divider 24 mayalso be provided to combine the two waves 13 and 14.

The waves caused to interfere with one another reach the optical sensorarrangement 20 which includes at least two sensors 21 and 22. As afunction of wave front differences caused by the hologram betweenmeasuring wave 13 and reference wave 14, different radiation intensitiesoccur at the two sensors 21 and 22. In signal processing arrangement 23,the absolute, two-dimensional position is determined from the differencein intensity by way of a comparison with a value stored in a memory. Forexample, the signal processing arrangement 23 calculates the signaldifference occurring at sensors 21 and 22 or it forms, for example, aratio therefrom. Greater resolution and/or measuring accuracy isattained if more than two sensors 21 and 22 are included in sensorarrangement 20. Sensor arrangement 20 is configured, for example, as anarray or a two-dimensional multi-sensor arrangement. In this case, aplurality of signals are available in signal processing arrangement 23which can be utilized for the absolute position determination.

In another embodiment, the absolute position is determined from thephase difference occurring between sensors 21 and 22. A determination ofthe phase relationships in the optical frequency domain is avoided ifeither the measuring wave 13 or the reference wave 14 is frequency orphase modulated relative to the other. In the end result, phase andfrequency modulation are the same. For this purpose, a modulator 16 isprovided which, for example, correspondingly modulates the referencewave 14. The phase relationship between the signals put out by sensors21 and 22 can be performed in signal processing arrangement 23 in thedomain of the modulation frequency of modulator 16.

Evaluation of the phase information has the significant advantage overan evaluation of the intensities that intensity changes which couldoccur, for example, due to the optical components becoming soiled or,for example, due to aging phenomena, do not influence the positiondetermination.

The complicated calculation of the hologram structure can be simplifiedif separate structures are predetermined for each one of the twodirections to be determined in the position measurement. An evaluationof the different structures is possible by the use of two optical beamsof different wavelengths.

According to a first modification of the apparatus according to theinvention, two measuring waves 13 and reference waves 14 are provided,with sensor arrangement 20 including at least two sensors for the onewave and at least two sensors for receiving the radiation at the otherwavelength. It is possible to have a completely separate structure forthe two measuring arrangements 11 for the two wavelengths. Preferably,some parts of the measuring structure 11 are used simultaneously forboth wavelengths. It is possible to guide both measuring beams 13jointly at different wavelengths, with the reference beams 14 beingdirected at an angle of about 90 degrees with respect to the projectionsof the reference beams 14 onto surface 18 for the two measuringdirections. It is also possible to divide the two measuring beams 13 forboth measuring directions and to guide the reference beams 14 jointly.Radiation source 12 may either produce the beams at differentwavelengths separately or jointly.

In a modification of measuring arrangement 11 employing two measuringwaves 13 and two reference waves 14, the wavelength of measuring wave 13and reference wave 14 is switched at predetermined time intervals. In afirst time period, the wavelength is generated for position measurementsin the one measuring direction and in a next time period the wavelengthis generated for the position measurement in the other measuringdirection. The particular advantage of this modification is that onlytwo sensors 21 and 22 are required. The necessary separate evaluation atthe different wavelengths is effected in signal processing arrangement23 which receives a signal that indicates the present wavelengths of themeasuring and reference waves.

The optical measuring arrangement 11 is preferably configured inintegrated form, with at least the measuring wave 13 and the referencewave 14 being conducted through a light waveguide 15. Particularlysuitable as the radiation source 12 is a semiconductor laser whichenables measuring arrangement 11 to have a particularly compactstructure.

Adjustment of the optical measuring arrangement 11 is limitedessentially to causing the measuring wave 13 and reference wave 14 tointerfere with one another. The angle of incidence of measuring wave 13and possibly of reference wave 14 with respect to hologram 18, on theone hand, and the observation angle of sensor arrangement 20 withrespect to hologram 18, on the other hand, are freely selectable withincertain limits.

We claim:
 1. An apparatus for making absolute two-dimensionalmeasurements of the position of an object with reference to a measuringarrangement comprising: means for directing an optical measuring waveonto a surface of the object, with said surface being configured as ahologram; means providing an optical reference wave for interfering withthe measuring wave diffracted by the hologram; an optical sensorarrangement comprising at least two sensors for detecting theinterfered-with waves; and a signal processing arrangement whichdetermines the position of the object from the output signals of thesensors and from a value stored in a memory.
 2. An apparatus accordingto claim 1, wherein the signal processing arrangement determines theabsolute position from sensor output signals which are proportional tothe intensity of the interfered-with waves.
 3. An apparatus according toclaim 1, wherein a phase or frequency modulation is provided between themeasuring wave and the reference wave, and wherein the signal processingarrangement determines the absolute position from the phase relationshipbetween the output signals of the sensors.
 4. An apparatus according toclaim 1, wherein the measuring and reference waves are directed fromdifferent directions onto a common point of incidence on the hologram.5. An apparatus according to claim 1, wherein the means providing anoptical reference wave directs the reference wave to a location not onthe surface of the object for interfering with the measuring wave afterthe measuring wave is diffracted at a point of incidence on the surfaceof the object.
 6. An apparatus according to claim 1, including asemiconductor laser for generating the measuring wave and referencewave.
 7. An apparatus according to claim 1, including an opto-acousticmodulator for modulating the measuring or reference wave.
 8. Anapparatus according to claim 1, wherein said optical sensor arrangementis a multisensor arrangement configured as an array.
 9. An apparatusaccording to claim 1, wherein said optical sensor arrangement includesat least four optical sensors provided in a two-dimensional sensorarrangement.
 10. An apparatus according to claim 1, wherein at least onefurther optical measuring wave is directed onto the surface of theobject, with said surface being configured as a hologram and at leastone further optical reference wave is provided for interfering with themeasuring wave diffracted by the hologram, with the further measuringand reference wave having a different wavelength compared to the firstmeasuring and reference wave and the measuring and reference wave at onewavelength is provided for the position measurement on the surface inone measuring direction and the measuring and reference wave at theother wavelength is provided for the position measurement on the surfacein another measuring direction, preferably a direction rotated by 90°;and wherein said optical sensor arrangement includes at least two groupsof at least two sensors, with each sensor group picking up a certainwavelength of the interfered-with waves.
 11. An apparatus according toclaim 1, wherein at least one further optical measuring wave is directedonto the surface of the object, with said surface being configured as ahologram and at least one further optical reference wave is provided forinterfering with the measuring wave diffracted by the hologram, with thefurther measuring and reference wave having a different wavelength thanthe first measuring and reference wave, and with alternatingly measuringwaves and reference waves exhibiting one wavelength and thereafter theother wavelength; and the measuring and reference wave of the onefrequency is provided for the position measurement in one direction andthe measuring and reference wave of the other wavelength is provided forthe position measurement on the surface in another direction, and asignal which in each case indicates the existing wavelength of themeasuring and reference wave is fed to the signal processingarrangement.
 12. An apparatus according to claim 1, realized inintegrated optical form.
 13. A method for making absolute,two-dimensional measurements of the position of an object with referenceto a measuring arrangement comprising the steps of: providing a surfaceof the object with a hologram containing absolute two-dimensionalposition information of the object; directing an optical measuring waveonto the surface of the object provided with the hologram; providing anoptical reference wave and causing same to interfere with the measuringwave diffracted by the hologram; detecting the interfered-with waveswith at least two sensors; and determining the position of the object bycomparing a value derived from respective output signals of the sensorswith a value stored in a memory.
 14. A method according to claim 13,wherein the step of determining includes determining the absoluteposition from sensor output signals which are proportional to theintensity of the interfered-with waves.
 15. A method according to claim13, further comprising the step of providing a phase or frequencymodulation between the measuring wave and the reference wave; andwherein the step of determining includes determining the absoluteposition using a value corresponding to the phase relationship betweenthe output signals of the sensors.